I created a folder on my Windows 7 host, called "Shared". On my Leopard (guest) I created a folder called "Windows" on the Desktop. I made sure to add the "Shared" folder as permanent under Shared Folders in Virtualbox's options.

Well, duh, of course it doesn't work because there are no GA for Mac OSX Guest system. If you actually bothered to check the ISO, you would see that it's lacking a .dmg file and even the release notes state it, as well as the manual.

Until there is Guest Additions for Mac OS X, you can still share folders on the host with the guest via the network. It is pretty simple:

1) Share a folder from the actual host machine (from the OS itself, not through the vbox shared folders settings)2) Use NAT for your guest's network settings3) From Mac OS X, Open "Finder" (looks like a square face, usually on the bottom-left)4) From the top menu, navigate to Go->Connect to Server5) For "Server Address" enter smb://10.0.2.26) Click "Connect"7) Enter your username and password (for the host machine)8) Select the shared folder from the list

This is the method I am using in my Mac OS X guests until the new Guest Additions is implemented.